Nuclear bodies (NBs) are distinct subnuclear domains present in both animal and plant cells. Although numerous NBs, such as Cajal bodies, PML (promyelocytic leukemia) NBs , and IGC (interchromatin granule cluster) have been extensively studied in mammalian systems, and changes in morphology and constitution of these nuclear bodies are associated with human diseases, the precise function and regulation of NBs are still poorly understood. One possible reason for this is the lack of genetic models to study NB functions. We propose to use phytochrome NBs in plant light signaling as a genetic model system to investigate general principles behind NB functions. Phytochromes are red and far-red photoreceptors regulating plant development and growth through transcription regulation. Phytochromes localize in the cytoplasm in the dark. Upon light activation, they relocate to the nucleus and form phytochrome NBs. We demonstrated that the formation of a photo-stable phytochrome B (PHYB) containing NBs is directly regulated by light and is tightly correlated to phytochrome responses. Based on these observations, we hypothesize that phytochrome NBs are directly involved in phytochrome signaling events. To test this hypothesis, we propose to: (1) identify and characterize new components required for PHYB-GFP NB formation by a confocal-based genetic screen. We have already identified twenty-nine such mutants. One locus, HMR, has been cloned, and two others rough-mapped. Strikingly, our preliminary studies show that the first gene (HMR) identified from this screen is required for both PHYB-GFP NB formation and early phytochrome signaling events including the light-dependent proteolysis of PHYA, a photo-labile phytochrome. Interestingly, HMR is structurally similar to the mammalian ortholog of a yeast protein RAD23, which is a multiubiquitin binding protein involved in protein degradation. These results provide the first genetic evidence linking phytochrome nuclear bodies with protein degradation, and demonstrate that this genetic screen will likely identify novel components linking phytochrome NB function and early phytochrome signaling events; (2) investigate the function of phytochrome NBs in early phytochrome signaling events. We propose a number of experiments to directly test whether phytochrome NBs are sites for PHYA degradation and/or transcription regulation; (3) define the function of HMR in PHYA degradation. We will test whether HMR acts as RAD23 by delivering PHYA to the proteasome for degradation. Collectively, the proposed experiments should contribute significantly to a better understanding of light signaling in plants. More importantly, they will also start to unravel general principles of NBs in cell signaling.